FIG. 1 illustrates a wireless communication system 10. The communication system has a plurality of base stations 121-12n (12). Each base station 12 communicates with wireless transmit/receive units (WTRUs) 141-14n (14) within its operating area or cell 161-16n (16). When a WTRU 14 is first activated, it is unaware of its location and with which base station 12 (or cell 16) to communicate. The process where the WTRU 14 determines the cell 16 to communicate with is referred to as “initial cell search.”
Initial cell search (ICS) in the proposed time division duplex mode of wideband code division multiple access, by way of example, typically comprises three steps. In step one, a WTRU searches for a primary synchronization code (PSC). Each base station transmits the same PSC in a primary synchronization channel (PSCH). The PSCH may be transmitted in one or two timeslots depending on the manner in which the system is implemented. In FIG. 2, the PSCH is shown being transmitted in two timeslots 20, 22. As shown in the exploded view of timeslot 22, each base station's PSC is offset in time according to a particular time offset. The time offset, typically expressed as particular number of signaling units called “chips,” is provided to reduce interference between secondary synchronization codes (SSCs) which are simultaneously transmitted along with each PSC by each base station in a system. The PSC that is selected by the WTRU is the one for which the WTRU has measured the highest power.
In step 2, the SSCs transmitted along with the selected PSC are used to determine the timeslot offset of the detected PSC and the code group. Then, in step 3, the scrambling code and unique midamble base identification number are determined.
This approach to initial cell search has drawbacks. One drawback is where a detected PSC leads to a cell belonging to a PLMN with which the WTRU may not communicate for whatever reason (e.g. a WTRU on a first network detecting a PSC leading to a cell of a second network wherein there is no shared-use agreement between the two networks). In this case, the WTRU will read the broadcast channel (BCH) of the unsuitable cell, realize that it is an unsuitable cell and begin another initial cell search. The problem is that while running subsequent initial cell searches, the WTRU often detects the same PSC and is led to the same unsuitable cell. This is inefficient in that the WTRU is wasting time and resources running additional initial cell searches for PSC's that lead to unsuitable cells and wasting additional time and resources reading the BCH of unsuitable cells and may eventually cause the initial cell search to fail based on a timeout.
It is therefore desirable to have a method and system for running initial cell search without such limitations.